In web alignment apparatus for multi-level web material such as carpeting, tufted fabrics and the like having a backing web and pile on the backing web spaced inwardly from the edge of the backing web, it is frequently desirable to maintain the edge of the pile in a preselected relation to an apparatus which operates on the web, independent of the lateral position of the edge of the backing web. This necessitates sensing the line of juncture between the backing web and the pile edge and this poses some problem if the backing web is such that it is not possible to pass either an air stream or a light beam through the backing web to sense the edge of the superposed face web or pile.
The U.S. Pat. No. 3,244,428 to Henderson discloses an electrical pile edge sensing apparatus which utilizes a plurality of contact fingers, some of which normally rest on the pile and others of which normally rest on the backing adjacent the pile, and which contact fingers operate switches to control positioning of the pile edge in accordance with the relative positions of the contact fingers. Such a pile edge sensing apparatus, however, can only effect a step type control and the pile edge can shift laterally a significant distance before one of the contact fingers either rides onto or off of the pile to change the electrical signal.
Pneumatic type pile edge sensing apparatus have also been heretofore made utilizing a tightly coiled spring as an air bleeder valve. The tightly coiled spring was rigidly supported at one end with its axis inclined at an acute angle to a longitudinal plane through the edge of the pile on the web, and a web engaging member rigidly attached to the other end of the coil spring and arranged to engage the pile edge so that the convolutions of the coil spring were spearated to variably open the valve when the web engaging member was either moved laterally by the pile edge or the web engaging member rode onto the pile. However, the convolutions of the spring which formed the bleeder valve also controlled the pressure exerted by the web engaging member on the web and pile edge and some difficulties were encountered in sensing the pile edge on webs which did not have a firm and well defined pile edge, such as occurs when the pile is thin, soft or loose. The web engaging member of the sensor sometimes tends to ride on top of the pile instead of sensing the pile edge and the prior coil spring type pneumatic sensor was not always sensitive enough to sense and produce an appropriate correction signal when the web engaging member rode on top of a thin, soft or loose pile.